Chronic diseases and impaired function related to physical inactivity are major factors in the disability, diminished quality of life, and burgeoning health care costs facing older Americans. While current physical activity training studies provide compelling evidence for the importance of regular physical activity to health and functioning in older adults, the vast majority of such studies have occurred in the controlled setting of the laboratory with small numbers of highly selected subjects. A critical next step is to extend such research to the community setting where the effectiveness of longer-term physical activity interventions for the broader American public can be more adequately determined. In addition, given the dearth of information related to enhancing quality of life in older adults, determining the efficacy of community-based physical activity programs m promoting changes relevant to quality of life issues is urgently needed. The objectives of this study are to: (1) conduct a telephone survey of randomly selected women and men equal to or greater than age 65 living in the city of Sunnyvale for purposes of obtaining a broadly representative subject sample; (2) determine the initial (6 months) and longer-term (one-year) effectiveness of a community-based regimen of physical activity in promoting significant changes in physical functioning and performance in this older adult sample; (3) evaluate the efficacy of the physical activity training regimen in promoting changes in functioning an well-being variables relevant to quality of life; and (4) use the study as an opportunity to enhance knowledge with respect to measurement of physical activity among older adults. The study design is a one-year randomized trial in which women and men recruited from a representative sample of older adults living in the community will be randomly assigned to one of three conditions: physical activity training; attention-control (Yoga/Stretching); or assessment-only control. Subjects in the two active conditions will receive year-long instruction involving a combination of class- and home-based training regimens. The training regimens are based on principles of social learning and social cognitive theories and designed to enhance long-term adherence and minimize injury. Data will be collected at baseline and at 6 and 12 months using a battery of age-relevant physical functioning and performance tests, a comprehensive set of rated functioning and well-being scales, and several promising measures of physical activity. Our primary hypotheses are that subjects assigned to physical activity training will demonstrate significant increases in physical functioning and performance, maintained through 12 months, relative to the other two conditions. Additional major questions of interest focus on the changes in rated functioning and well-being among the three conditions across the 12-month period, and comparisons across different physical activity assessment instruments. This study will significantly contribute to efforts aimed at moving the older American population closer to the National Health Objectives for the Nation for the Year 2000.